1. Field of the Invention
This invention relates to plasma enhanced etch processes. More particularly, this invention relates to plasma etch processes useful in etching thick layers of silicon nitride for printhead cartridges, inkjet printers, surgical instruments, bio--medical devices and other articles where silicon nitride structures are shaped or formed.
2. Description of the Related Art
Silicon nitride is a desirable compound for structures such as printhead cartridges and surgical devices because it retains its shape and is resilient. However, the very characteristics that make silicon nitride a material of choice are the characteristics that make most silicon nitride fabrication processes difficult and time consuming.
Silicon nitride has been used in integrated circuit devices as an etch stop or masking layer, a protective layer, and a dielectric layer. The high durability of silicon nitride also makes it useful in fabrication or micro-machining of small devices such as surgical instruments, biomedical implements and printer cartridges. Compared to integrated circuit devices, the amount of silicon nitride material required in the above listed micro-machining applications is often orders of magnitude greater than the material required for integrated circuits. For example, the thickness of a passivation layer of silicon nitride in an integrated circuit device may be on the order of hundreds of angstroms (1 .ANG.=1.times.10.sup.-10 m). In contrast, in a typical micro-machining application the thickness of a silicon nitride layer may be on the order of tens of microns or even more than one hundred microns. As a result, etching methods suitable for use in integrated circuits and relatively thinner silicon nitride layers may prove impractical for use with substantially thicker silicon nitride layers.
One conventional method of etching silicon nitride is wet etching. In wet etch processes, the workpiece having the material to be etched is loaded into a chemical bath. The composition of the chemical bath is selected to remove the exposed materials. Phosphoric acid and hydrogen fluoride are commonly employed to etch silicon nitride. Wet etch methods are necessarily isotropic which may be undesirable when forming precise shapes or shapes with small features. Additionally, chemical baths are dangerous because of potential exposure to reactants. Wet etch rates can also vary as the etched materials alter the chemical make-up of the bath. The variable chemical make-up results in etch rate variations from batch to batch.
Conventional dry etch methods, also known as reactive ion etching, offer several improvements over wet etch methods. Reactive ion etch processes are more repeatable than the chemically variable bath. This repeatability of the etch process thereby increases the likelihood that each workpiece in a batch is exposed to the same etch process. In this manner, repeatability and manufacturability of the etch process is improved.
Regardless of whether conventional dry or wet etch methods are employed, removal rates for silicon nitride are not commercially viable for micro-machining applications. Here, micro-machining applications refer to those applications requiring the removal of about five microns of material, to applications which remove between about 10 to 50 microns of material. Some applications entail the removal of over 100 microns of material. The problem of etching such large amounts of material is brought into specific relief when one considers that etch rates in both chemical wet etch baths and reactive ion etch processes produce etch rates on the order of hundreds of angstroms per minute.
For example, U.S. Pat. No. 4,793,397 entitled "Selective Thin Film Etch Process" (issued to Dunfield et al. on Dec. 27, 1988) describes silicon nitride etch processes which utilize plasmas formed from mixtures of SiF.sub.4 and O.sub.2 ; NF.sub.3, SiF.sub.4 and O.sub.2 ; and NF.sub.3, SiF.sub.4, He and O.sub.2. According to Dunfield, these plasmas provide silicon nitride etch rates in "a useful range of 0-500 .ANG. per minute" and a "preferred range of 100-200 .ANG./minute." Almost ten years later on Jul. 28, 1998, U.S. Pat. No. 5,786,276 entitled "Selective Plasma Etching of Silicon Nitride In Presence of Silicon or Silicon Oxides Using Mixture of CH.sub.3 F or CH.sub.2 F.sub.2 and CF.sub.4 and O.sub.2 " issued to Brooks et al. According to Brooks, "high SiN etch rates are defined as at least about 1000 .ANG./min. and . . . even more preferably at least about 2500 .ANG./min."
The etch rates described by Brooks and Dunfield may be suitable for integrated circuit device applications where silicon nitride thicknesses are generally less than 2 microns and are more likely to be only a few hundred angstroms. In contrast, micro-machining applications generally have silicon nitride layers greater than five microns thick while some applications have silicon nitride layers ranging in thickness from about 15 microns to about 50 microns. Still other micro-machining applications require machining silicon nitride layers over 100 microns thick. The etch rates provided by the previously described etch methods would result in unreasonably long processing times. Such long processing times would hinder manufacturing processes of articles with thick (i.e. greater than 5 microns) silicon nitride structures to such an extent that commercial fabrication of silicon nitride structures and articles becomes impracticable.
Thus, what is needed is a method of plasma etching thick silicon nitride layers for micro-machining applications that overcomes the shortcomings of the prior art to provide higher, commercially viable silicon nitride etch rates.